AppRunner CloudWatch Alarm not showing metrics

I'm trying to create an alarm for AppRunner. My AppRunner service is running fine.

It keeps showing "Insufficient data". I'm using Terraform, but weirdly enough, creating an alarm manually also results in the same no data message. I've tried memory metrics but same thing. Any ideas?

resource "aws_cloudwatch_metric_alarm" "apprunner_cpu" {
  alarm_name                = "<redacted>"
  comparison_operator       = "GreaterThanOrEqualToThreshold"
  evaluation_periods        = 2
  metric_name               = "CPUUtilization"
  namespace                 = "AWS/AppRunner"
  period                    = 60
  statistic                 = "Average"
  threshold                 = 80
  alarm_description         = "AppRunner CPU utilization"
  insufficient_data_actions = []

  dimensions = {
    ServiceName = aws_apprunner_service.main.service_name
    ServiceID   = aws_apprunner_service.main.service_id
  }
}

No alarms are being triggered:

However it seems that data is fine: